This invention relates to production of aluminum, and more particularly it relates to an improved carbon cathode for use in the production of aluminum.
In the Hall-Heroult process for making primary aluminum, aluminum oxide is dissolved in a molten salt such as cryolite and then electrolyzed to form molten aluminum at the cathode. The electrolysis is called out at a temperature in the range of about 930 to 980.degree. C. The molten salt is contained in a steel shell which is lined with refractories and carbonaceous material. The lining containing the cathode metal, located in the bottom of the cell, is usually made of carbon materials. In addition, refractories are used to maintain thermal conditions in the cell. The amount of carbon used is substantial. For example, a Hall-Heroult cell of moderate size uses about 24,000 pounds of carbon block for lining purposes and uses about 10,000 pounds of carbon ramming paste to complete the lining and to hold the carbon blocks in place. The cell has to be relined about every 4 to 6 years, producing large quantities of used carbonaceous material and refractories, i.e., spent potlining.
The use of carbonaceous cathodes is not without problems. For example, they are not readily wettable with molten aluminum. Thus, conductivity through the surface of the cathode is not uniform but tends to be intermittent. Also, the carbon cathode surface reacts with the molten aluminum to form aluminum carbide which depletes the cathode at a rate of 2 to 5 cms per year for an operating electrolytic cell. This depletion is fostered by the presence of sludge containing fluoride bath components at the interface between cathode carbon and metal. The aluminum carbide also is detrimental because it results in a high electrical resistivity material which interferes with the efficiency of the cell.
The carbon cathodes have another problem. That is, sodium and lithium penetrate or intercalate the carbon blocks causing them to swell. In addition, the sodium results in the formation of sodium cyanide causing disposal problems with the spent potlinings. The Environmental Protection Agency has listed spent potlinings as a hazardous material because they contain cyanides. Thus, it will be seen that there is a great need for a carbonaceous cathode that is wettable with molten aluminum, which is resistant to formation of cyanide, and shows a decreased tendency to swell due to sodium intercalation.
In attempts to provide aluminum wettable surfaces on carbon cathodes, application of titanium boride or zirconium boride has been suggested. These materials have been used as tiles to cover the cathode surface and are described in U.S. Pat. Nos. 3,400,061; 4,093,524; 4,333,813; and 4,341,611. However, these approaches have not been without problem. That is, the tiles and coatings tend to fall off after a short period of use, and this interferes with continued use of the cell. Also, coatings of titanium diboride have been applied in cement to the carbonaceous surface in U.S. Pat. Nos. 4,544,469; 4,466,692; 4,466,995; 4,466,996; 4,526,911; 4,544,469 and 4,624,766. EPO 0 021 850 suggests electroplating titanium diboride onto the carbon surface. U.S. Pat. No. 5,028,301 suggests deposition of a coating composed of titanium diboride and titanium carbide on cathode parts from supersaturated dissolved elements in electrowon aluminum. In a book entitled "Cathodes in Aluminum Electrolysis", 2nd edition, published 1994 by Aluminium-Verlag and authored by M. S.o slashed.rlie and H. A. .O slashed.ye limited durability and cost of the material are cited as obstacles to effective industrial use.
With respect to disposal of spent carbonaceous material, numerous approaches have been used to convert the cyanide or to render the spent potliner safe for disposal. For example, U.S. Pat. No. 5,222,448 discloses that spent potliner is treated by introducing it into a vessel, and exposing it to the heat of a plasma torch at a temperature of at least 1000.degree. C. As a result, carbon is gasified and converted to combustible carbon monoxide or hydrocarbons, or to carbon dioxide; inorganic material is melted to form slag; fluoride compounds are melted, vaporized, or reduced to gaseous HF; cyanide compounds are destroyed; and all other materials, including sulfur compounds, are either melted or gasified. As a result, the spent potliner is rendered non-hazardous, and the quantity of remaining slag has both its solid volume and mass substantially reduced by a factor of at least 1.5:1 in mass and at least 3:1 in volume relative to the input spent potliner.
U.S. Pat. No. 4,576,651 discloses a process for treating fluoride-contaminated scrap lining material from electrolytic reduction cells which comprises mixing the material with 7-30 pales of sulfuric acid and sufficient water to bring liquid content to 60-80 pairs per 100 parts of lining material, mixing in sufficient lime to at least neutralize the sulfuric acid and make the slurry slightly alkaline, the slurry then being allowed to set into a solid mass. The slurry should be of a paste-like consistency. The lime may be wholly calcium hydroxide, but a substantial proportion may be in the form of calcium carbonate. The scrap, before or after the above treatment with lime and sulfuric acid, is preferably heated to 150-500.degree. C. in the presence of water vapor to destroy cyanides.
U.S. Pat. No. 4,763,585 discloses a process for the combustion of ground, spent potlinings generated during the production of metallic aluminum. The process includes grinding the potlinings to a particle size of not greater than about 2 inches in any dimension; mixing with the ground potlinings from about 1 to about 20 wt. % based upon the weight of the potlinings, of a powdered inert additive having a median particle size of not greater than 10 micrometers, and burning the ground potlinings in a combustor at a temperature in the range of from 1400.degree. F. to about 2200.degree. F., the additive coating the ground potlinings and preventing their agglomeration in the combustion zone therein.
U.S. Pat. No. 4,973,464 discloses a method for removal of cyanides from spent potlinings from aluminum manufacture. The method discloses the treatment of ground, spent potlinings generated during the production of metallic aluminum to reduce cyanide content to environmentally nonhazardous levels. Potlinings are ground or otherwise suitably reduced in size to a particle size of not greater than about 2 inches in any dimension and roasted in a stream of air or nitrogen at a temperature between about 500.degree. and 1400.degree. F. Roasting for an appropriate time-temperature interval reduces cyanide content to desired levels without combustion of a major portion of carbonaceous material, resulting in an end product rich in carbon and fluorine which may be salable because of this content.
U.S. Pat. No. 4,993,323 discloses that an environmentally acceptable and effective method for thermal destruction of Spent Potliner (SPL) by Fluidized Bed Combustion (FBC) has been established. This method has overcome problems associated with ash agglomeration, ash leachate character and emission control, the primary obstacles for applying FBC to the disposal of SPL and like solid fuels. Specifically, "recipes" of appropriate additives (fuel blends) are proposed. A mixture of lignite, limestone and SPL in an appropriate proportion has proven to notably increase the agglomeration temperature of the ash, allowing this low-melting waste to be destroyed continuously by FBC. Ash leachate character is modified by control of ash chemistry to ensure that fluoride anions and metallic cations are at or below acceptable limits.
U.S. Pat. No. 5,024,822 discloses a process for treating spent potlining from the electrolytic smelting of aluminum in cryolite including incinerating the potlining to combust carbonaceous material to form an ash at a temperature low enough to maintain low fluorine vapor pressures, admixing siliceous material with the potlining either before or after the ash-forming stage, and heating the ash and siliceous material to form a glassy residue.
In Norwegian Disclosure 175, 159, the cyanide-containing potlining is treated is situ by raising the cell temperature before shut-down of the cell, thus promoting penetration of electrolyte into the lining to react with the cyanide.
However, it will be noted that these treatments are post-treatments to correct the hazardous waste problems resulting from spent potlinings, and most of them are relatively expensive. Thus, it will be seen that there is a great need for a method that permits the use of carbonaceous cathodes but is effective in promoting wetting with molten aluminum and is effective in preventing formation of undesirable compounds such as cyanide compounds during use of the cell to produce aluminum. Promoting wetting of the cathode greatly increases the efficiency of the cell and by preventing formation of compounds such as cyanide compounds, any post-treatment for spent cathodes can be greatly simplified.